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Published June 20, 2008 | Published
Journal Article Open

The optical spectra of 24 μm galaxies in the COSMOS field. I. Spitzer MIPS bright sources in the zCOSMOS-bright 10k catalog


We study zCOSMOS-bright optical spectra for 609 Spitzer MIPS 24 μm-selected galaxies with S_(24) μ m > 0.30 mJy and I < 22.5 (AB mag) over 1.5 deg^2 of the COSMOS field. From emission-line diagnostics we find the following: (1) SFRs derived from the observed Hα λ6563 and Hβ λ4861 lines underestimate, on average, the total SFR by factors of ~5 and 10, respectively. (2) Both the Calzetti et al. and the Milky Way reddening laws are suitable to describe the extinction observed in IR sources in most cases. (3) Some IR galaxies at z < 0.3 have low abundances, but many others with similar IR luminosities and redshifts are chemically enriched. (4) The average [O III] λ5007/Hβ λ4861 ratios of ν L^(24 μ m)_ν > 10^(11) Lסּ galaxies at 0.6 < z < 0.7 are ~0.6 dex higher than the average ratio of all zCOSMOS galaxies at similar redshifts. Massive star formation and AGNs could simultaneously be present in those galaxies with the highest ionizing fluxes. (5) Roughly 1/3 of the galaxies with metallicity measurements at 0.5 < z < 0.7 lie below the general mass-metallicity relation at the corresponding redshifts. The strengths of the 4000 Å break and the Hδ EW of our galaxies show that secondary bursts of star formation are needed to explain the spectral properties of most IR sources. The LIRG and ULIRG phases occur, on average, between 10^7 and 10^8 yr after the onset of a starburst on top of underlying older stellar populations. These results are valid for galaxies of different IR luminosities at 0.6 < z < 1.0 and seem independent of the mechanisms triggering star formation.

Additional Information

© 2008. The American Astronomical Society. Received 2007 December 5; accepted 2008 March 6. This paper is based on observations made with VIMOS on the Melipal-VLT telescope, undertaken at the European Southern Observatory (ESO) under Large Program175.A-0839.Also based on observations made with the Spitzer Observatory, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. We thank our referee Jonathan Gardner for useful comments and suggestions.We are grateful to Gustavo Bruzual and Stephane Charlot for making available the latest version of their synthetic SED template library.

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